The present invention relates generally to joining metal to graphite, and particularly to forming a bus-bar connection between a pair of graphite electrodes for use in an electrochemical cell.
Graphite is used in many industrial fields, including chemical, electrical, metallurgical, electrochemical, nuclear, and rocket fields. In several of these areas of manufacture, it is desirable to join metal to graphite. In the field of electrochemistry, graphite is widely used as an electrode material due to its electrical and thermal characteristics, and because it is one of the most inert materials with respect to chemical reactions. Furthermore, in this field, suitable metals having low electrical resistivity are used as a bus-bar material for joining two or more graphite electrodes together. In this particular application, it is important to achieve a low transition or contact resistance between the metal bus-bar and graphite electrodes being joined in order to minimize voltaic losses.
One such electrochemical application is the zinc-chloride battery, where graphite is employed for both the positive and negative electrodes. During the charging of the battery, zinc metal is electrodeposited on the negative or zinc electrode and chlorine gas is generated at the positive or chlorine electrode from an aqueous zinc-chloride electrolyte. During the discharging of the battery, the reactions are reversed to generate electricity from the terminals of the battery. The zinc electrode is constructed from dense or fine grained graphite, and the chlorine electrode is constructed from a liquid permeable porous graphite.
One technique of joining metal to graphite is described in U.S. Pat. No. 4,100,332, issued on July 11, 1978, entitled "Comb Type Bipolar Electrode Elements And Battery Stacks Thereof", and is herein incorporated by reference. In this patent, it is taught to provide for a press or interference fit between the graphite electrodes and the graphite or valve metal bus-bar wall. Accordingly, the electrodes are constructed slightly thicker than the grooves in the bus-bar, so that when pressed in they may be retained by a pressure fit. It is also stated that the electrodes may be connected to the bus-bar by cementing, plasma spraying at the point of contact, or welding.
Another zinc-chloride battery stack design is described in U.S. Pat. No. 4,071,660, issued Jan. 31, 1978, entitled "Electrode For A Zinc-Chloride Battery And Batteries Containing The Same", and is herein incorporated by reference. In this patent, it is taught to provide an interference fit between graphite electrodes and a titanium bus-bar in one instance, and a cemented connection between graphite electrodes and a titanium bus-bar in another instance. It should also be noted that in addition to the cement and interference fit connections, bolt connections have also been employed to join metal to graphite.
The present invention provides a novel method of joining metal to graphite which results in a low transition or contact resistance. Particularly, the method comprises: providing a plurality of openings in a graphite member to be joined, interposing the graphite member between two metal members and positioning the metal members to cover the openings in the graphite member, and resistance spot welding the metal members to the graphite member at the openings in the graphite member. The resistance spot welding may be accomplished with one or more pairs of opposing welding electrodes for a sequential or simultaneous spot welding at the openings in the graphite member. The welding electrodes are used to apply a predetermined amount of pressure to the metal members at the opening selected for spot welding, and sufficient electrical current is passed through the welding electrodes to spot weld the metal members together through the opening.
During the resistance spot welding, at least a portion of the metal from the two metal members flows into the opening of the graphite member under the pressure of the welding electrodes, and the gap between the metal members is bridged. As the metal cools, it contracts thereby exerting a force drawing the two metal members together to achieve a good electrical contact with the graphite member. Some penetration of the metal into the pores of the graphite member also occurs.
In the zinc-chloride battery application, it is preferred that titanium or tantalum be used for the metal members. These particular metals have a relatively low electrical resistivity compared to graphite, and are generally chemically resistant or inert to the zinc-chloride electrolyte and other chemical entities with which they will come into contact. Although titanium is extremely reactive above 1000.degree. F. in an oxygen and nitrogen atmosphere, it has been found that no special atmospheric shielding is required for the resistance spot welding.
Other features and advantages of the present invention will become apparent in view of the drawings and the following detailed description of the preferred embodiment.